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Around 150 AD, the Hellenistic astronomer Claudius Ptolemy described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux, all of which are clearly of orange or red hue.[1] The discrepancy was first noted by amateur astronomer Thomas Barker, who prepared a paper and spoke at a meeting of the Royal Society in London in 1760.[2] The existence of other stars changing in brightness gave credence to the idea that some may change in colour too; Sir John Herschel noted this in 1839, possibly influenced by witnessing Eta Carinae two years earlier.[1]Thomas Jefferson Jackson See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.[1] He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus as colouring the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus' poem Phaenomena.[1]Seneca, too, had described Sirius as being of a deeper red colour than Mars.[3] However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus.[1] It is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.[4][5]

In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola — "reddish". The authors proposed this was further evidence Sirius B had been a red giant at the time.[6]

This is a real visual image of the red giant Mira by the Hubble Space Telescope. Credit: Margarita Karovska (Harvard-Smithsonian Center for Astrophysics) and NASA.

This is a visual or optical image of Aldebaran. Credit: Aladin at SIMBAD.{{fairuse}}

This image shows the Hyades star cluster, the nearest cluster to us with Aldebaran the bright star on the left. Credit: NASA, ESA, and STScI.{{free media}}

A red giant is a luminous giant star The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are the so-called red giant branch stars (RGB stars). Another case of red giants are the asymptotic giant branch stars (AGB). To the AGB stars belong the carbon stars of type C-N and late C-R. The stellar limb of a red giant is not sharply-defined, as depicted in many illustrations. Instead, due to the very low mass density of the envelope, such stars lack a well-defined photosphere. The body of the star gradually transitions into a 'corona' with increasing radii.[7]

In Hindu astronomy Aldebaran is identified as Rohini ("the red one"), one of the twenty-seven daughters of Daksha and the wife of the god Chandra (the Moon).

Aldebaran shown on the left is a red giant on the red giant branch (RGB) at 44 times the diameter of the Sun.[8] equivalent to approximately 61 million kilometres.

Aldebaran is a slow irregular variable star, type LB, varying by about 0.2 in apparent magnitude from 0.75 to 0.95.[9] With a near-infrared J band magnitude of −2.1, only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter.[10]

Its photosphere shows abundant carbon, oxygen, and nitrogen.[11] With its slow rotation, Aldebaran may lack a corona and may not be a source of hard X-ray emission. However, small scale magnetic fields may still be present in the lower atmosphere, resulting from convective turbulence near the surface. (The measured strength of the magnetic field on Aldebaran is 0.22 Gauss (G).[12]) Any soft X-ray emissions from this region may be attenuated by the chromosphere, although ultraviolet emission has been detected in the spectrum.[13] The star is currently losing mass at a rate of (1–1.6) × 10−11 M⊙ yr−1 (about one Earth mass in 300,000 years) with a velocity of 30 km s−1.[11] This stellar wind may be generated by the weak magnetic fields.[13]

Beyond the chromosphere of Aldebaran is an extended molecular outer atmosphere where the temperature of 1,000−2,000 K is cool enough for molecules of gas to form. This region lies between 1.2 and 2.8 times the radius of the star. The spectrum reveals lines of carbon monoxide, water, and titanium oxide.[11] Past this radius, the modest outflow of the stellar wind itself declines in temperature to about 7,500 K at a distance of 1 AU. The wind continues to expand until it reaches the termination shock boundary with the hot, ionized interstellar medium that dominates the Local Bubble, forming a roughly spherical astrosphere with a radius of around 1,000 AU, centered on Aldebaran.[14]

Red supergiants (RSGs) are supergiant stars (luminosity classI) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive. Betelgeuse and Antares are the best known examples of a red supergiant. These stars have very cool surface temperatures (3500–4500 K), and enormous radii. The five largest known red supergiants in the Galaxy are VY Canis Majoris, VV Cephei A, V354 Cephei, RW Cephei and KW Sagittarii, which all have radii about 1500 times that of the [S]un (about 7 astronomical units, or 7 times as far as the Earth is from the [S]un). The radius of most red giants is between 200 and 800 times that of the [S]un. Absolute luminosities may reach -10 magnitude compared to +5 for our [S]un.

This Hertzsprung-Russell diagram shows the evolution of stars of different masses. The red clump is marked RC on the green line showing the evolution of a star of 2 solar masses. Credit: .

The red clump is a feature in the Hertzsprung-Russell diagram of stars. The red clump is considered the metal-rich counterpart to the horizontal branch. Stars in this part of the Hertzsprung-Russell diagram are sometimes called clump giants. These stars are more luminous than main sequence stars of the same surface temperature (or colder than main sequence stars of comparable luminosity), or above and to the right of the main sequence on the Hertzsprung-Russell diagram.

Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red giant branch stars in a galaxy to gauge the distance to that galaxy. It has been used in conjunction with observations from the Hubble Space Telescope to determine the relative motions of the Local Cluster of galaxies within the Local Supercluster. There is a sharp discontinuity in the evolutionary track of the star on the HR diagram.[15] This discontinuity is called the tip of the red giant branch. When distant stars at the TRGB are measured in the I-band, their magnitude is somewhat insensitive to their composition of elements with more mass than helium (metallicity) and their mass. This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (Population II).[16]

This mosaic image taken by the Hubble Space Telescope of Messier 82 combines exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

"The presence of ERE has been established spectroscopically in ... the starburst galaxy M82 (Perrin, Darbon, & Sivan 1995)."[17]

"This mosaic image [at right] is the sharpest wide-angle view ever obtained of M82. The galaxy is remarkable for its bright blue disk, webs of shredded clouds, and fiery-looking plumes of glowing hydrogen blasting out of its central regions."[18]

"Throughout the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way Galaxy. The resulting huge concentration of young stars carved into the gas and dust at the galaxy's center. The fierce galactic superwind generated from these stars compresses enough gas to make millions of more stars."[18]

"In M82, young stars are crammed into tiny but massive star clusters. These, in turn, congregate by the dozens to make the bright patches, or "starburst clumps," in the central parts of M82. The clusters in the clumps can only be distinguished in the sharp Hubble images. Most of the pale, white objects sprinkled around the body of M82 that look like fuzzy stars are actually individual star clusters about 20 light-years across and contain up to a million stars."[18]

"The rapid rate of star formation in this galaxy eventually will be self-limiting. When star formation becomes too vigorous, it will consume or destroy the material needed to make more stars. The starburst then will subside, probably in a few tens of millions of years."[18]

"Located 12 million light-years away, M82 appears high in the northern spring sky in the direction of the constellation Ursa Major, the Great Bear. It is also called the "Cigar Galaxy" because of the elliptical shape produced by the oblique tilt of its starry disk relative to our line of sight."[18]

"The observation was made in March 2006, with the Advanced Camera for Surveys' Wide Field Channel. Astronomers assembled this six-image composite mosaic by combining exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments."[18]